1. Field of the Invention
The present invention concerns the deposit of enamel on a glass substrate, especially on a window. It concerns, in particular, a new process for fabrication of an enamelled layer on a glass substrate. It also concerns an enamel composition to be used for fabrication of a layer on a glass substrate.
2. Discussion of the Background
By enamel is meant, according to the invention, both the enamel composition, or paste, used for the deposit, and the enamel layer formed at different stages of the fabrication process, as described below.
Enamels are well known in the state of the art and are used, in particular, on glass substrates, such as vehicle or building windows or mirrors. Enamels are also used to form borders, decorative layers, protective layers, especially for adhesive layers used for mounting windows, e.g., windshields or rear windows, in body sashes, or for mounting accessories, e.g., rear-view mirror support bases, or as protective layers for collector strips of heating systems.
The enamel used for these applications is generally formed from a powder containing a glass frit, metallic oxide-based pigments used as coloring agents, a medium, also called a vehicle, generally containing a binding agent allowing adhesion of the enamel to the glass during enamel deposit, and a solvent or organic diluting agent allowing the desired viscosity to be reached for application of the composition on the substrate.
This application can be carried out using various procedures, such as spraying, silk-screening, coating with a roller, etc. Use is preferably made of silk-screening, which makes it possible to obtain quite varied and reproducible shapes and layer structures.
To manufacture the enamelled layer, enamel is preferably deposited on the substrate by silk-screening. The moist layer thus formed is dried until this layer possesses adhesion to glass and general resistance sufficient to ensure that, as required, the window can be handled without the appearance of marks on the layer formed. Other layers may potentially be deposited, then dried, and the enamelled layer or layers thermally treated at high temperature in order to vitrify the enamel and obtain the final coating. This high-temperature vitrification treatment is generally associated with the thermal treatment used to transform the glass sheet, for example for bending the glass sheet into a convex shape.
Drying of the enamel corresponds either to the removal of the solvent or diluting agent from the medium, infrared (IR) heating being generally used for this purpose; or hardening by polymerization using UV radiation, depending on the enamel composition employed. These types of drying operations have disadvantages. For example, IR drying wastes energy. It also requires an extended drying line, and further poses the problem of the evaporation of organic solvents and of environmental protection.
UV drying can be used only for an enamel containing organic compounds which are polymerized under the effect of the UV radiation, and, in particular, acrylic compounds. The problem of the harmfulness of these products is once again raised. Furthermore, it was recently shown that the use of UV radiation led to the production of ozone. In addition, polymerization using UV radiation can be used only for the manufacture of very thin enamel layers, which are generally too thin to be used as the enamelled border of an automobile window, for example, since they are not sufficiently opaque.
The present invention remedies the disadvantages cited. It provides a new procedure for manufacture of at least an enamel layer over at least one part of a glass substrate, in particular a window. This procedure is fast, saves energy, and reduces environmental risks.
In addition to the advantages already indicated, since microwave energy is preferably absorbed by the enamel composition but only to a limited extent by the glass substrate, energy savings are realized; in addition, because of the resulting limited rise in the temperature of the substrate, the final substrate-cooling phase, which often proves necessary during an infrared drying operation, can be eliminated, or at least shortened.